1. Field of the Invention
The present invention relates to semiconductor packaging technologies, and more particularly, to semiconductor packages which contain multiple semiconductor dice and/or substrates.
2. Description of the Related Art
Very Large Scale Integrated (VLSI) semiconductor dice are usually housed in semiconductor packages. Normally, one semiconductor package contains only one die.
There are three conventional types of semiconductor packages. Molded plastic packages contain a lead frame molded within a plastic body. A lead frame is a sheet metal framework having several electrical leads and a Die Attach Pad (DAP) serving as a principal mounting surface (or seating plane) upon which a die is mounted. The die may be bonded either directly to the DAP or to a substrate attached to the DAP. The electrical leads provide an electrical path from inside the molded plastic to outside the plastic. Some common types of molded plastic packages are: Plastic Chip Carrier (PCC), Molded Dual Inline Package (MDIP), Plastic Quad Flat Pack (PQFP), Small Outline (SO), Shrink Small Outline Package (SSOP), Transistor Outline Package (TO), Very Small Outline Package (VSOP), and Thin Small Outline Package (TSOP).
The second conventional type of semiconductor package is the cavity package. In the cavity package, a cavity base which serves as a principal mounting surface (or seating plane) upon which a die is mounted is contained within a hollow housing. Unlike the molded plastic package, the die in a cavity package is surrounded by air. Several electrical leads provide an electrical path from inside the housing to outside the housing. Some common types of cavity packages are ceramic packages, metal cans, plastic packages, and any combination thereof.
The third conventional type of semiconductor configuration is the Chip-On-Board (COB) assembly. In the COB, a die is directly bonded to a circuit board or substrate which serves as a principal mounting surface (or seating plane). The die is usually covered and protected with a plastic material. A variety of different types of electrical leads may be employed to provide an electrical path from inside the plastic material to outside the plastic material.
Although the three conventional types of semiconductor packages have various different shapes and sizes, each of them includes several electrical leads and a principal mounting surface (or seating plane) upon which a die is mounted.
A common method of making electrical connections between the die and the electrical leads, which is used in each of the three conventional types of packages, is wire bonding. Wire bonding is a method of making electrical interconnections among components in a discrete package by means of fine wire conductors welded to the individual components. Thus, a fine wire conductor has one end connected to an electrical lead and the other end connected to an electrical contact on the die. Wire bonding is a popular method of interconnecting dice. Improvements in capillary design, wire bonding process control, and wire properties have allowed finer pitch bonding to be made.
Often two or more semiconductor dice are electrically interconnected to provide a single circuit assembly. Under the one die per package paradigm, the interconnection of two or more dice requires enough physical space for an equal number of packages. In order to decrease size and weight, as well as improve device performance, there have been several attempts to combine two or more dice into a single package. In the high density integrated circuit packaging industry, the combination of two or more dice in a single package is typically referred to as a Multi-Chip Module (MCM) or Multi-Chip Package (MCP). Although the terms Multi-Chip Module and Multi-Chip Package have slightly different meanings, for purposes of this discussion, they may be used interchangeably.
The most common MCM is the "side-by-side" MCM. In this version two or more dice are mounted next to each other (or side by side each other) on the principal mounting surface of either a plastic molded package, cavity package, or COB assembly. The die may be mounted directly to the principal mounting surface or it may be mounted on a substrate material which is itself mounted directly to the principal mounting surface. Interconnections among the dice and electrical leads are commonly made via wire bonding.
The side-by-side MCM, however, suffers from a number of disadvantages. Laying out the dice side by side on the principal mounting surface within a molded plastic package or a cavity package is not the most optimal way to use package real estate. Such real estate is preciously limited since, in most cases, the dice have to fit within some standard form factor previously designed for only one die. If the dice are not properly laid out, the real estate restriction will limit the number of dice that can be incorporated into the MCM. Furthermore, unoptimized dice layout yields correspondingly unoptimized wire bonding resulting in wire cross over, long wire lengths and small wire-to-wire separation. Wire cross over, where one wire loops over another wire, is highly undesirable because shorting may occur as a result of molding conditions. Similarly, long wire lengths and small wire-to-wire separation can pose high risks for wire sweep under fast mold transfer conditions or high resin viscosity.
Other attempts at building MCMs have involved placing two or more dice on top of one another and then securing the "stack" of dice in a package. Currently available stacked MCMs are fabricated by stacking entire wafers and then sawing the stacked wafers into stacked dice. Thus, each of the individual die in a particular stack is the same size.
One disadvantage of currently available stacked MCMs is that they are all memory devices; it is believed that no mixed technology devices are currently available in stacked form. Another disadvantage of currently available stacked MCMs is that they require unique and specialized packages. Furthermore, complex and expensive methods are used to make electrical interconnections among the dice; the methods of interconnection currently used are Controlled Collapse Chip Connection (C4) and Tape Automated Bonding (TAB).
Controlled Collapse Chip Connection (C4), also known as "flip chip", involves the use of a large number of solder bumps on a die surface which allow it to be bonded face down. Among the advantages are improved thermal performance, electrical characteristics and reworkability. On the other hand, commonly acknowledged disadvantages include requirements for precise alignment, difficulties in cleaning and inspection, uniform solder joint height for all connections to be made and a substrate with low coefficient of thermal expansion for longer thermal cycle life. Furthermore, in order to use C4, all solder bumps and interconnections must be implemented before and during the stacking of the dice; in other words, after the dice are stacked, no additional interconnections can be made.
Tape Automated Bonding (TAB) refers to a process whereby dice are joined by patterned metal on polymeric tape using thermocompression bonding. Subsequent attachment to a substrate or board is carried out by outer lead bonding. Tape Automated Bonding (TAB) has seen only limited application to MCMs. Although TAB possesses many advantages, barriers to its wide usage include the high starting cost of custom tape, the moisture sensitivity of polyimide tape and the need to switch to single point bonding with large dies to circumvent planarity issues.
Thus, there is a need for a low cost MCM which overcomes the disadvantages of currently available MCMs.